The American Chemical Society researchers, funded by the National Natural Science Foundation of China and others, were inspired by plants living on the seafloor as they set out to create flexible triboelectric nanogenerators (TENGs).
Minyi Xu, Zhong Lin Wang and colleagues wanted to copy the way strands of seaweed vibrate, allowing them to charge bendable triboelectric surfaces and convert wave movement into electricity.
In some coastal areas, networks of sensors collect information on currents, tides and visibility, to help ships navigate and monitor water quality. This ‘marine internet of things’ is mostly powered by batteries that occasionally have to be replaced, which is time-consuming and expensive. Wind and solar power could be used, but the researchers said they are not suitable for underwater applications.
Looking to harness the ocean’s continuous movement as a renewable energy source, the team initially developed floating devices that converted wave energy into electricity using rotating magnets. These devices were inefficient with less frequent waves, however, such as those found underwater.
The researchers turned to TENGs – which rely on surfaces coming in contact to produce static electricity – because of their effectiveness for harvesting low-frequency, low-amplitude wave energy.
To make the triboelectric surfaces, the researchers coated 3.8cm by 7.6cm strips of two different polymers in a conductive ink. A small sponge was then wedged between the strips, creating a thin air gap, and the whole unit was sealed.
As the TENGs were moved up and down in water during tests, they bent back and forth to generate electricity. When the researchers put them in water pressures similar to those found underwater in coastal zones, they found the air gap between the two conductive materials decreased. The devices still generated a current at 100kPa of pressure, however — the same pressure that typically exists at a 9m water depth, where there is almost no underwater wave movement.
Finally, the team used a wave tank to demonstrate that multiple TENGs could be used as a small underwater power station, supplying energy for either a thermometer, 30 LEDs, or a miniature lighthouse LED beacon. The researchers said their seaweed-like TENG could reduce the reliance on batteries in coastal zones, including for marine sensors.
The research was published in ACS Nano.
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